Alkylation is a reaction in which an alkyl group is added to an organic molecule. Thus an isoparaffin can be reacted with an olefin to provide an isoparaffin of higher molecular weight. Industrially, the concept depends on the reaction of C.sub.2 to C.sub.5 olefin with isobutane in the presence of an acidic catalyst producing a so-called alkylate. This alkylate is a valuable blending component in the manufacture of gasolines due not only to its high octane rating but also to its sensitivity to octane-enhancing additives.
Industrial alkylation processes have historically used concentrated hydrofluoric or sulfuric acid catalysts under relatively low temperature conditions. Acid strength is preferably maintained at 88 to 94 weight percent by the continuous addition of fresh acid and the continuous withdrawal of spent acid. As used herein, the term "concentrated hydrofluoric acid" refers to an essentially anhydrous liquid containing at least about 85 weight percent HF.
Hydrofluoric and sulfuric acid catalyzed alkylation processes share inherent drawbacks including environmental and safety concerns, acid consumption, and sludge disposal. For a general discussion of sulfuric acid alkylation, see the series of three articles by L. F. Albright et al., "Alkylation of Isobutane with C.sub.4 Olefins", 27 Ind. Eng. Chem. Res., 381-397, (1988). For a survey of hydrofluoric acid catalyzed alkylation, see 1 Handbook of Petroleum Refining Processes 23-28 (R.A. Meyers, ed., 1986).
Hydrogen fluoride, or hydrofluoric acid (HF) is highly toxic and corrosive. However, it is used as a catalyst in isomerization, condensation, polymerization and hydrolysis reactions. The petroleum industry used anhydrous hydrogen fluoride primarily as a liquid catalyst for alkylation of olefinic hydrocarbons to produce alkylate for increasing the octane number of gasoline. Years of experience in its manufacture and use have shown that HF can be handled safely, provided the hazards are recognized and precautions taken. Though many safety precautions are taken to prevent leaks, massive or catastrophic leaks are feared primarily because the anhydrous acid will fume on escape creating a vapor cloud that can be spread for some distance. Previous workers in this field approached this problem from the standpoint of containing or neutralizing the HF cloud after its release.
U.S. Pat. No. 4,938,935 and U.S. Pat. No. 4,985,220 to Audeh and Greco, as well as U.S. Pat. No. 4,938,936 to Yan teach various methods for containing and/or neutralizing HF acid clouds following accidental releases.
But it would be particularly desirable to provide composition which avoids the cloud forming problems associated with HF while providing commercially useful activity as an isoparaffin-olefin alkylation catalyst. Diluents and complexing agents for hydrofluoric acid have, in the past, been disclosed for various purposes as noted in the following references.
U.S. Pat. No. 2,615,908 to McCaulay teaches thioether-HF-copper complex compounds and a method for preparing the same. Potential uses for the thioether-HF-copper composition compounds are listed from column 6, line 55 through column 8 at line 3. The method is said to be useful for purifying HF-containing vent gases from an industrial HF alkylation plant. See column 7, lines 10-24.
U.S. Pat. No. 3,531,546 to Hervert discloses a HF-CO.sub.2 catalyst composition which is said to be useful for alkylation as well as olefin isomerization.
U.S. Pat. No. 3,795,712 to Torck et al. relates to acid catalysts comprising a Lewis acid, a Bronsted acid, and a sulfone of the formula R--SO.sub.2 --R', where R and R' are each separately a monovalent radical containing from 1 to 8 carbon atoms or form together a divalent radical having from 3 to 12 carbon atoms.
U.S. Pat. No. 3,856,764 to Throckmorton et al. teaches an olefin polymerization catalyst comprising (1) at least one organoaluminum compound, (2) at least one nickel compound selected from the class consisting of nickel salts of carboxylic acids, organic complex compounds of nickel, or nickel tetracarbonyl and (3) at least one hydrogen fluoride complex prepared by complexing hydrogen fluoride with a member of the class consisting of ketones, ethers, esters, alcohols, nitriles, and water.
U.S. Pat. No. 4,636,488 discloses an anhydrous nonalcoholic alkylation catalyst comprising a mixture of a mineral acid and an ether in proportions of from about 50 to about 99 weight percent of mineral acid and from about 1 to about 50 weight percent of ether. Useful mineral acids include HF; see column 4 at lines 56-60.
Promoters such as alcohols, thiols, water, ethers, thioethers, sulfonic acids, and carboxylic acids are disclosed in combination with strong Bronsted acids such as HF, fluorosulfonic and trihalomethanesulfonic acids in U.S. Pat. No. 3,778,489 to Parker et al. The promoters are said to modify the activity of the strong Bronsted acids for alkylation.
U.S. Pat. No. 3,795,712 to Torck et al. teaches hydrocarbon alkylation in the presence of a sulfone and from 10.sup.-5 to 5 moles of hydroiluoric acid per liter of sulfone.
U.S. Pat. No. 4,025,577 and U.S. Pat. No. 4,094,924 to Siskin et al. teach isoparaffin-olefin alkylation catalysts comprising a hydrogen halide and a metal fluoride, and, optionally, a suitable diluent.
The preceding references demonstrate the desirability of a liquid Bronsted acid catalyst (such as HF) for isoparaffin-olefin alkylation, as well the utility of liquid Bronsted acids in combination with metal halides, particularly metal fluorides. However, metal fluorides have been found to cause operational problems in two principal areas. First, the corrosovity of metal halides toward materials of process unit construction is of a character sufficiently distinct from that of the strong Bronsted acids that more costly preventive measures (including alloy selection, coating, and additive treatment) are required. Second, the presence of a metal halide in a liquid alkylation catalyst composition complicates process design and increases capital and operating costs for the catalyst recovery, treatment, and recycle facilities. Thus, it would be highly desirable both from the standpoint of initial process design and unit construction, as well as from the standpoint of operational simplicity and reliability, to provide an alkylation catalyst composition and process which avoids both the safety and environmental concerns associated with concentrated HF while also overcoming the design and operational difficulties attendant to the use of intentionally added metal halides. Allowed U.S. application Ser. No. 07/856,270, filed Mar. 23, 1992, disclosed a method for decreasing the corrosivity of mixtures of HF and sulfolane comprising adding a controlled amount of water to the HF/sulfolane mixture. U.S. application Ser. No. 07/719,274, now abandoned, disclosed mixtures of strong Bronsted acids and solvents having Donor Numbers of less than about 40. Mixtures of hydrofluoric acid and one or more of these solvents were particularly promising as safer alternatives to concentrated HF for isoparaffin-olefin alkylation, but were found to be corrosive toward carbon steel.